Shuttle orbiter Endeavour was again ready to launch to the International Space Station. However, poor weather again put pay to the fifth launch attempt for STS-127, moving launch to Wednesday. Meanwhile, the results of the Ground Umbilical Carrier Plate (GUCP) investigation appear to have been validated via Sunday’s leak-free “near-complete duration” tanking.

STS-127 Fifth Attempt Scrub:

With two delays caused by a leak of Endeavour’s External Tank (ET-131) at the GUCP, followed by two weather related delays Endeavour was running out of time to make her trip to the ISS – prior to standing down for the Russian Progress launch.

The weather was once again an issue for Monday’s attempt, with forecasts noting a 60 percent constraint to launch by the time STS-127’s countdown reaches T-0. Also, rain during the launch countdown would be very problematic for an exposed tyvek cover – as noted in attempt 4’s article.

With the delay to Wednesday, the Rotating Service Structure (RSS) will be placed back around the vehicle for a replacement of the cover.

Water intrusion into the thruster holds the threat of freezing during ascent, which could cause problems for its operation on orbit. However, it was the weather in the local area for around T-0 that resulted in another scrub being made official.

Several “funnies” had also been listed at the Mission Evaluation Room (MER) at the Johnson Space Center (JSC), ranging from GSE (Ground Support Equipment).

The main notes on the MER list related to findings by the Ice Inspection Team – which had arrived at the pad to look at all areas of the shuttle after tanking had entered stable replenish.

During the hold ahead of T-9 minutes, a waiver was issued to allow the vehicle to proceed to launch.

Another issue related to a Freon (coolant) loop in Pad 39A’s Ground Coolant Loop Resevoir Level. However, this is a known condition, as an existing GSEPR (Problem Report) on the freon leak was noted.

A problem with a chiller pump at the pad was also not going to be a cause any problems for the remainder of the count.

While Wednesday was originally unavailable due to the Progress launch, NASA had appeared to have used an option to negotiate the extension of STS-127’s window by a day with the Russians, and/or reduce mission content in order to have a final opportunity available on Thursday – if approved by the MMT.

GUCP Status (images to be added shortly):

One benefit of Sunday’s – and indeed Monday’s – scrub was the confirmation the GUCP troubleshooting plan appears to have worked, with no leaks detected throughout the countdown, up to the point of polling to come out of the T-9 minute hold.

This provided additional confidence – following the tanking test at the start of the month – that the engineering work on correcting what was an alignment issue of just 0.357 degrees in the counter-clockwise direction, has been successful.

The amount of work that went into fixing the issue can be seen in a 47 page presentation to the all-powerful Program Requirements Control Board (PRCB), dated July 7 and acquired by L2.

“The objective of the overview – to identify the configuration that allows us to proceed to a launch attempt with very high confidence of success.”

Engineers were tasked with checking historical issues with the hardware, in order to attempt a root cause analysis of not only ET-131’s leak during STS-127’s opening two countdowns, but the leak on the opening launch attempt of STS-119 with ET-127.

They were also asked to present the worst case outcome of the potential scenarios surrounding a problem with the GUCP hardware.

“Identified 21 scenarios using inputs from community, new fault tree, timelines. Collected evidence to support/refute each scenario,” added the presentation. “11 scenarios are fully (4) or partially (7) mitigated by the actions taken. Evidence reviewed by team ruled out 10 scenarios.

“Worst case effect/risk vary with individual scenario: Could result in an improper function of the vent line (leakage of flammable hydrogen external to the ET), resulting in a launch scrub.

“2-piece flight seal has greater resiliency and provides additional capability for misalignment. 2-piece seal tested to 0.050”. Concentricity and other measurements during assembly show minimal motion of GUCP. Successful tanking test (50 ppm GH2). Note -successful tanking has never been followed by a leak resulting in a scrub. Tanking test observations show minimal motion of GUCP feet.”

The modifications noted will now be implemented on ET-132 (for STS-128) and all downstream tanks that are yet to arrive at the Kennedy Space Center (KSC) from their manufacturing base – the Michoud Assembly Facility (MAF) in New Orleans.

Although this forward plan included flight rationale to the Mission Management Team (MMT) for STS-127’s tanking and launch, the investigation will continue, given the findings do not provide a clear root cause for STS-119/ET-127’s leak.

This will include further work at the test facility at the Marshall Space Flight Center (MSFC).

“ET-131 mitigations do not invalidate certification or hazard controls. Continue investigation to determine the sensitivity of this interface to off-nominal geometry for future tanks, other contributing factors, and finalize long-term changes to preclude leaks.

“Seal edge inspections showed that the compression band formed on the seal edge by bellows probe contact was sporadically inconsistent around the entire circumference of the seal. Areas of wide, rolled, jagged, thin & wavy contact were all observed.

“Dents and gouges were also evident around the entire circumference of the seal edge.”

Given ET-131’s first flight seal only suffered from a few rolled edges – likely caused by the misalignment – ET-127’s leak is probably related to a specific fault with its flight seal, which in turn adds confidence ET-131’s issue was mainly via the now-solved misalignment problem.

For live updates of launch attempt 5 – refer to the live update pages, linked above.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.